The present invention relates to a vehicle control system and more particularly relates to a method for improving the control behavior of an electronically regulated and/or controlled driving stability control system. A brake system for implementing the method is also an object of the present invention.
German published application DE 373 1 756 A1, for example, discloses a method for controlling the driving stability of an automotive vehicle. In this method for controlling the driving stability of an automotive vehicle, a variation of the limit values of the yaw velocity, which is influenced additionally by the vehicle speed and the coefficient of friction, is evaluated in dependence on the steering angle. Based on this variation of limit values, criteria are defined which initiate braking pressure introduction and/or braking pressure modulation.
Precharging is necessary in the prior art methods of controlling the- driving stability of an automotive vehicle in order to achieve a satisfactory system dynamics, especially at low temperatures. This precharging function is performed, for example, by an electrically operable (active) brake force booster or a hydraulic precharging pump. Because the viscosity of the brake fluid rises at low temperatures, this measure is necessary in order to ensure a reliable and constant application of the method for controlling the driving stability of an automotive vehicle event at low temperatures.
Therefore, an object of the present invention is to improve upon a method for controlling the driving stability of a vehicle of the above-mentioned type to the end of obviating the need for precharging. Operability, even at low temperatures, shall be ensured without restrictions nonetheless. Another object of the present invention is to provide a brake system to implement the method for controlling driving stability.
This object is achieved by the present invention in that criteria for evaluation of the driving situation and for the early detection of a driving situation inhering an increased risk are derived from the rotational behavior of the wheels, and in that the commencement of driving stability control is prepared upon detection of a driving situation inhering an increased risk. Evaluating and deriving criteria of a driving situation due to the wheel rotational behavior may take place with a remarkable time advantage compared to the usually employed sensors, such as a yaw rate sensor or a transverse acceleration sensor because the reaction of the vehicle chassis to driving maneuvers and driving situations is sensed by the sensors. In comparison thereto, the reaction of the vehicle wheels is available at a much earlier point of time, and the considerably shorter time constant is decisive for a possibly following control action.
In a preferred embodiment of the present invention, criteria for evaluating the driving situation can be defined by recognition of differences in the rotational behavior of the individual wheels. The early detection of a driving situation inhering an increased risk by way of detecting differences in the rotational behavior of the individual wheels permits utilizing another criterion. In addition, a threshold of intervention may be derived from the criteria found. This is e.g. possible by a computer-assisted further processing operation on the basis of predetermined fixed or variable models of the wheel rotational speeds determined by the wheel rotational speed sensors.
Advantageously, the commencement of driving stability control is prepared when the intervention threshold is exceeded. A signal for the commencement of driving stability control may be transmitted from a computer which computes the intervention thresholds to the controller electronics and/or controller hydraulics by way of a suitable interface.
When the intervention threshold is exceeded, pressure increase in a hydraulic brake system can be prepared. This may be carried out by switching on a hydraulic pump, for example, which is already provided in a brake system of this type in most cases.
A preferred embodiment of a brake system for implementing the method of controlling the driving stability of an automotive vehicle may include hydraulically operated wheel brakes and a hydraulic pump with a clocked actuation.
The clock frequency of the hydraulic pump may favorably be increased when the intervention threshold is exceeded. This permits a softer control which is advisable for comfort reasons.
Further details and advantages of the present invention can be taken from the following description and the illustrations of a preferred embodiment of the method for controlling the driving stability of an automotive vehicle according to the present invention.